1. Field of the Invention
The present invention relates to ladder filter devices and elastic wave resonators.
2. Description of the Related Art
In the prior art, ladder filter devices are used as, for example, band pass filters of RF circuits for cellular phones or the like. An example of such ladder filter devices is described, for example, in Japanese Unexamined Patent Application Publication No. 09-246911. In this exemplary ladder filter device, a parallel arm resonator is composed of an apodization-weighted surface acoustic wave resonator, and a series arm resonator is composed of a regular surface acoustic wave resonator. Japanese Unexamined Patent Application Publication No. 09-246911 describes that the ladder filter device with the foregoing configuration is capable of suppressing ripples in a pass band.
However, there is a problem that the ladder filter device described in Japanese Unexamined Patent Application Publication No. 09-246911 cannot adequately suppress an insertion loss at a low frequency side portion of pass band.